Instructions
Win the eight badges of the Indigo League. Beat Brock in Pewter City, Misty in Cerulean City, Erika in Celadon City, Lt. Surge in Vermilion City, Blaine in Cinnabar Island, Sabrina in Saffron City and Giovanni in Viridian City. Each of the trainers will give you a badge after you defeat them.
Go to and enter the Indigo Plateau. Travel through Route 23 to get there. It will be to the east of Mt. Silver and to the north of Victory Road.
Battle and defeat the Elite Four. Make sure you have a variety of different types of Pokemon to battle with, as the Elite Four will use Ice, Fighting, Ghost and Dragon-type Pokemon against you.
Go to the Pokemon Center in Cinnebar Island and talk to Bill. He will then take you to the Sevii Islands. His friend Celio will ask you to get sapphire and ruby while you are there. This is the Sevii Island quest.
Make your way through the Sevii Islands, responding to any puzzles and challenges you encounter. You will find the ruby on the road in One Island, and you will win the sapphire by defeating the scientist on the Island with the Rocket Warehouse. Give the ruby and sapphire to Celio to complete the Sevii Island quest. By returning the ruby and sapphire, you will make the Network Machine functional again.
Capture 60 different Pokemon, at least, before going to the Cerulean Cave.
Go to the Cerulean Cave in the North-West Corner of Cerulean City in Kanto. The guy blocking the front of the cave will be gone.